The present invention is generally directed to a structure for mounting a heat-generating semiconductor, such as a triac type semiconductor, onto a heat dissipating device, or heat sink.
Triac semiconductors have, in recent years, been used with increasing frequency in various electric and electronic components. Such semiconductor devices generate large quantities of heat which must be dissipated in order to prevent the deterioration and, ultimately, the destruction of the semiconductor device. It is known to mount such semiconductors onto heat dissipating devices, such as "heat sinks" which, because of the high heat conductivity thereof, are often formed of an electrically conductive metal plate, to keep the semiconductor temperature within acceptable limits. A typical triac switching device is formed with a body portion containing the semiconductor and an apertured metallic mounting tab extending therefrom. Such a triac is also usually provided with a plurality of electrical leads extending from the opposite side of the body portion from the mounting tab.
Because of the high heat generated by triacs, numerous mounting methods have been developed to assure heat dissipation therefrom. A variety of such mounting techniques have been the subject of patents including U.S. Pat. No. 3,641,474 which teaches the utilization of a mounting screw through the mounting tab to connect the semiconductor to a heat sink. One problem encountered in the use of a mounting screw is that, as the screw is tightened, the body portion containing the semiconductor tends to rotate, causing the protruding electrical leads to twist, possibly causing a short circuit. The teaching of this patent is that the twisting problem can be eliminated by providing anti-twisting tabs. However, another problem of this mounting method results from the fact that the mounting tab of the triac is often electrically live and must be electrically isolated from the mounting screw. Moreover, tightening of the mounting screw has been found to bend the tab, lifting the body portion away from the heat sink, thereby reducing the heat dissipation possible.
Another method of mounting such heat-generating semiconductor devices is illustrated in U.S. Pat. No. 4,259,685 wherein a metal spring is associated with the semiconductor device to bias it into good heat conductive relationship with the associated heat sink. However, the use of such springs can result in other problems such as the possibility of the springs breaking or falling out of the housing and short circuiting the triac or other portion of the electronic assembly. This is in addition to the extra assembly care that must be taken to assure the spring is electrically insulated from the conductive parts of the semiconductor.
Still another method of mounting such heat-generating semiconductors to a heat sink is illustrated in U.S. Pat. No. 4,199,654 in which a semiconductor housing is provided having a body cavity arranged to accept the semiconductor device. In the device disclosed in this patent, the body cavity of the housing has a depth which is slightly greater than the thickness of the body portion of the triac so that all of the force is applied to the mounting tab in an attempt to assure the necessary good contact between the semiconductor and the heat sink to assure adequate heat removal. However, it has been found that it is possible for the tab to be bent slightly so that, although the tab itself is in good contact with the heat sink, the semiconductor body itself is held out of contact with the heat sink due to the force applied to the tab. This is contrary to good thermodynamic practice because the heat generated in the semiconductor body must be conducted away by the tab rather than directly from the semiconductor body into the heat sink.
Yet another method of attaching a heat-generating semiconductor to a heat sink is disclosed in U.S. Pat. No. 4,449,292 in which the semiconductor is applied to the heat sink by means of soldering or the use of an adhesive. While in theory this may provide a satisfactory solution to the problem, in practice it has been found that these methods of attaching the semiconductor are less than satisfactory in many cases because the adhesive or solder joint may be degraded by the heat generated, which reduces the conductivity of the joint, thus increasing the temperature, eventually destroying the joint and the semiconductor.
While the problem of heat removal from heat-generating semiconductors is significant because it can result in the destruction of the semiconductor, it has been found that a far more serious problem results when such a semiconductor fails due to the smoke and soot generated by the failure, which can seriously damage or destroy the entire power supply in which it is incorporated. As a result, not only must the semiconductor device be replaced but the entire power supply must also be cleaned, repaired or replaced, significantly increasing the cost of such a failure. Experience has shown that, with prior art methods of assembling such heat-generating semiconductors, the failure rate has ranged from one to six percent per month, obviously an undesirable and expensive situation.